A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic apparatus, the substrate may typically be supported on a substrate table. Prior to the transfer of the pattern to the substrate, the substrate may be inspected and/or measured in a metrology area. Subsequently, the substrate is transferred by the substrate table to an area in which the pattern is transferred to the substrate. In order to ensure that the pattern that is transferred to the substrate is accurately positioned relative to the substrate and/or other patterns already formed on the substrate, it is desirable to know the position and movement of the substrate during the measurement and/or inspection process and during the image transfer process to a high degree of accuracy. This may be achieved by measuring the position of the substrate relative to the substrate table on which it is supported and subsequently monitoring the position and/or movement of the substrate table.
In order to measure the position and/or movement of the substrate table, known systems typically utilize a target mounted on one of the substrate table and a reference frame of the lithographic apparatus and a sensor mounted on the other of the substrate table and the reference frame that can measure the position or movement of the target relative to the sensor. However, the desired range of movement of the substrate table is relatively large because the substrate table should be able to move between an area in which any part of the substrate can be measured and/or inspected and an area in which a pattern may be transferred to any part of the substrate. Furthermore, in some lithographic apparatus, two substrate tables are provided such that while one substrate, supported on a first substrate table is having a pattern transferred to it, another substrate, supported on the second substrate table may be being inspected and/or measured. In such apparatus, the desired range of movement of the substrate tables is even larger because additional space is provided such that the substrate table supporting a substrate that is being inspected and/or measured can be transferred to the area in which a pattern may be transferred to the substrate and the substrate table supporting the substrate that has had a pattern transferred to it should be able to move successively to a position in which the substrate is unloaded from the substrate table, a position in which a new substrate is loaded to the substrate table and a position in which the new substrate is inspected and/or measured. In other words, the two substrate tables should be capable to swap over.
The larger the range of movement of the substrate table, the harder and/or more expensive it becomes to provide a system to measure the position and/or movement of the substrate table to the desired high level of accuracy. In particular, in systems in which the target for the position sensor is mounted to the substrate table, the larger the range of movement, the more complicated the system and the more difficult it becomes to maintain the desired accuracy. For systems in which the target is static and mounted to, for example, the reference frame, the larger the range of movement of the substrate table, the larger the target should be. Providing large targets may be expensive because it is generally difficult to make large targets that have the desired accuracy over their full range. Accordingly, increasing the range of movement of the substrate table, significantly increases the cost of the system to measure the position and/or movement of the substrate table.